A major design consideration of a gas turbine is the thermal and mechanical stresses due to the connection between the ductwork and the pipes that penetrate the duct. Specifically, the need to accommodate the insertion of hot pipes through relatively cool inlet and exhaust exterior duct walls. If the piping system does not fully accommodate all relative movements, the pipe penetration point eventually may fail. Gas turbine ducts often are required to be penetrated with pipes to supply steam for cooling, to provide oil for lubrication, or for other many other purposes. The pipe penetration must be designed to contain very high temperature, high velocity, and poisonous exhaust gases. If a leakage point in the duct occurs, the leak could lead to hazardous conditions for plant personnel and for the surrounding turbine equipment. The failure of the duct due to the structural characteristics of the duct materials also is a concern.
The relative movement of the pipe (in three dimensions) may be about two (2) to about four (4) inches (about 5.1 to about 10.2 centimeters) or so as compared to the duct. This design condition generally happens more severely during thermal transients where the pipe quickly expands relative to the duct due to internal heating of the penetrating pipe and insulation protecting the exterior shell. In addition, the pipe is hotter than its connection point to the duct such that the connection also needs to be thermally compliant.
A penetrating pipe is generally welded to a slightly larger pipe stub that protrudes from the duct wall. This type of connection works well as long as there are no appreciable thermal differences between the penetrating pipe and the pipe stub or appreciable movement/growth of the penetrating pipe upstream of the penetration. To address the thermal stress issue, designers generally locate the highest mechanical stress points away from the maximum thermal stress points by encasing the penetrating pipe in a larger cylindrical or conical shroud that is insulated and welded to the ductwork and to the pipe. These designs can accommodate some degree of thermal discrepancies, but add complications to the turbine system as a whole by requiring pipe loops and expansion joints to minimize the relative movement at the pipe penetration.
Thus, there is a desire for a pipe penetration system that can accommodate large axial, vertical, and radial displacements during operation of the gas turbine. The pipe penetration system should contain the internal air or gas flows yet be flexible enough to accommodate thermal and mechanical stresses. Such a system should be economical and should be able to be installed without special machining or tooling so as to result in a simplified piping system.